Constant tension system



June 2, 1959 M. E. HILLMAN CONSTANT TENSION SYSTEM 3 Sheets-Sheet 1 Filed Jan. 10, 1955 n. 0 m M mm ,0 ME 4 1m M o c m M 9 c 6 V. B \lllllllllllllllllllllllllllllJ 3 Sheets-Sheet 2 Fig.3

/ liili M. E. HILLMAN CONSTANT TENSION SYSTEM 230 GEN June 2, 1959 Filed Jan. 10, 1955 INVENTOR.

Malcolm E. Hillmon .M. E. HILLMAN CONSTANT TENSION SYSTEM June 2, 1959 3 Sheets-Sheet 5 Filed Jan. 10, 1955 INVENTOR. Malcolm E. Hillmon BY 417 Fig.4

United States Patent CONSTANT TENSION SYSTEM Malcolm E. Hillman, Seattle, Wash., assignor of onethird to Archibald G. Montgomery and one-third to Harold E. Lloyd, both of Seattle, Wash.

Application January 10, 1955, Serial No. 480,709

5 Claims. (Cl. 318-6) This invention relates to a constant tension system and more particularly, to such a system for maintaininga substantially uniform tension in a cable, hose or the like. For ease of description, the object tensioned will be hereinafter referred to as a tow-rope or a cable. The inventions many other applicable uses, two examples of which are the tensioning of a re-fueling hose, either for air-borne or water-borne vessels, and tensioning a rescue line, should be self-evident. My reference to tow-rope tensioning is to be given a liberal interpretation as meaning these and other applicable uses to which the invention may be placed.

In certain industries such as the paper and steel industries tension control systems for regulating the tension on the paper and steel strips are at the present time widespread. Some of these systems incorporate an amplidyne circuit having an amplidyne generator supplying the field coils of a generator in a motor-generator couple, i.e., the generator is electrically coupled to the motor and drives the same, thereby, the amplidyn regulates the output of the motor employed for driving the paper or steel strip in a particular section of the manufacturing process. These systems comprise a tensiometer riding on the strip of either the paper or steel and which transmits an electric signal to the amplidyne generator, said signal being indicative of the tension on the strip. This amplidyne generator in response to the signal adjusts the exciting field on the main generator so as to vary the output of the drive motor for either reducing or increasing the tension on the strip. Although such a system employing a tensiometer riding on a strip of material traveling in one direction is successful in the steel and paper industries it is not so successful when applied to maintaining constant tension in a towing cable which is employed for towing a log boom or a barge behind a tugboat-or one ship by another ship. In such a system employing a tensiometer riding on a cable there is an inherent electrical error of from five to fiteen percent depending on the circuit components and the adjustments in these components causing the motor to either be continuously heaving-in or paying out the cable. Additional error is introduced by the tensiometer and permanent deformation of the tensiometer beam and, when the system is used on board ship, erratic ship movements causing cable whip with the tension in the cable decreasing or increasing and a consequent heaving-in or paying out of the cable.

In View of the limitations of the constant tension system incorporating both a tensiometer and an amplidyne generator for controlling a motor-generator couple I have perfected an improved constant tension system comprising a motor for driving a winch and a control means for regulating the torque output of the motor. Said torque output is proportional to the current flowing through the motors armature and said current depends on whether the electromotive force developed by the motor adds to or opposes the voltage applied to the motor by the generator. By having the control means regulate the voltage applied to the motor it is possible to maintain a susbtantially uniform 2,889,502 Patented June 2, 1959 current fiow through the armature and a nearly constant torque output because the effective voltage impressed on the motor, the sum of the applied voltage and the generated by the motor, does not vary even though the applied voltage and the generated may supplement each other or oppose each other.

Accordingly, it is an object of this invention to provide a dynamoelectric responsive system capable of maintaining a constant tension in a cable and the like, such as strips of material, undergoing variations in tension.

As a further object this invention aims to provide a constant tension system requiring fewer electrical components than known existing constant tension systems require.

A further object is the providing of a constant tension system whose deviation from a predetermined tension is determined by an electric control means coupled to a driving motor.

An additional and important object is the provision of -a constant tension system which is both economical to manufacture and operate.

With yet additional objects and advantages in view which, with the foregoing, will appear and be understood in the course of the following description and claims,

the invention consists in the new system and in the novel construction and in the adaptation and combination of parts hereinafter described and claimed:

In theaccompanying drawings:

Figure 1 is a schematic wiring diagram of one embodiment of my constant tension system.

Fig. 2 is a schematic wiring diagram of another embodiment of my constant tension system, this embodiment incorporating means for maintaining the cable at a certain position.

Fig. 3 is a schematic wiring diagram of my constant tension system as employed on a shipboard installation; and

Fig. 4 is the wiring diagram of my constant tension system as employed on a shipboard installation.

The essence of my invention is a control means for controlling the voltage applied to a driving motor so as to maintain a constant eifective voltage on the armature of the motor and thereby a constant armature current flow and uniform motor torque output. The effective voltage is the resultant of the voltage applied to the motor and the voltage generated by the motor. In certain situations the motor will generate an opposing the applied voltage and in other situations the motor will generate an complementing the applied voltage. Before proceeding to fully present my constant tension system there will be an introductory word concerning both an amplidyne generator and a motor-generator couple as the amplidyne generator is a specific embodiment, for ease of description, of one of many control means for controlling the torque output of the motor in the motorgenerator couple.

In the general field of dynamoelectric machines the most nearly complete and spontaneous control over the mechanical output of a motor is realized when the motor is a D.-C. machine supplied with power from a specifically designed D.-C. generator. The generator in turn may be separately excited and direct control over its performance determined in the field circuit. Since the generator may necessitate a plurality of field windings in order to produce the desired output characteristics, the field windings in which external control is exerted are referred to specifically as the control fields. In controlling the generator precise control is generally associated With the expenditure of only a small amount of power in the controlling circuitry and with the use of reasonably sensitive measuring equipment to provide the controlling signal. One generator having control fields that require only small excitation currents and therefore small amounts of control power, the ratio of the armature power being controlled to the control field power, is an amplidyne or amplidyne generator.

In an amplidyne a control field located in the direct axis, or load axis, produces a flux which gives rise to a speed voltage across quadrature brushes located in the quadrature axis, or sometimes referred to as the shortcircuit axis. As the quadrature axis brushes are shortcircuited it is possible for a relatively large current to exist in the quadrature-axis brush circuit for a relatively small current in the control field. The quadrature-axis current produces cross-magnetizing armature reaction as in a conventional D.-C. generator. This armaturereaction flux is stationary in space and centered in the quadrature axis of the machine, and gives rise to a speed voltage across load brushes located in the load axis. These direct-axis or load axis brushes supply the load. Also in the amplidyne is a direct-axis compensating field which is essential to the maintenance of voltage when load current is drawn as in the absence of the compensating field armature-reaction magneto-motive force, created by the load current, would directly oppose the control-field magneto-motive force resulting in substantial collapse of machines fluxes and voltages. However, the armaturereaction magneto-motive force created by the load current is compensated as exactly as possible by the load current flowing through the compensating winding.

As is seen the excitation of the amplidyne is furnished by both the armature and the field with the control field responsible only for the production of suflicient voltage across the quadrature-axis brushes to force the quadrature current through the low-resistance short-circuited path.

In the amplidyne several signals associated with the controlling process may be impressed simultaneously and superimposed on each other in the direct-axis magnetic circuit of the machine, and the magnetic eflect of any one signal'may be added to or subtracted from the combined eflect of the other signals. In a typical amplidyne circuit a voltage, whose magnitude is proportional to the desired value of the quantity being controlled is impressed on one of the control field coils and the resulting field is called a reference field. On another control field, connected so as to oppose the reference field, there is impressed a voltage proportional to the quantity being controlled. When the desired and actual values are of the same magnitude and opposed to each other the output voltage of the amplidyne is essentially Zero, and when the two values are difierent the output voltage of the amplidyne is approximately proportional to the difference and of a polarity corresponding to this difference. With the amplidyne it is possible to have a number of control fields, some in opposition and some in augmentation of the reference field, so that output voltage of the amplidyne is a resultant of all of these control fields.

Proceeding now to describe the invention for maintaining a uniform tension on a cable 9 it is seen that the same comprises a motor-generator set 10, anamplidyne circuit 11, a tachometer generator circuit 12, a tension-setting circuit 13, and a prime mover 14. The motor-generator set comprises a D.C. generator 15 and a D.C. motor 16, such as a series motor, and connecting with the motors shaft 17 is a gear box 18, a shaft 20 and a winch 21 on which is wound the cable 9. Associated with the motor is a field coil circuit 22 having a coil 23 with an approximately constant current flow therethrough, said current being supplied by a constant voltage source such as a battery 24 and associated with the generator is the amplidyne circuit comprising an amplidyne 25 and the generators field coil 26. The main generator and the amplidyne generator are both driven by the prime mover 14. The tension-setting circuit comprises a constant voltage source such as a battery 27, a variable rheostat 28, and a control field 30 on the direct axis of the amplidyne. And, the tachometer generator circuit comprises a tachometer generator 31 coupled to the main D.C. motor by, for example, a chain and sprocketdrive with a sprocket on the motors shaft, a sprocket on the tachometer generators shaft, and a chain interconnecting the two sprockets, so that when the motor rotates the tachometer correspondingly functions, and a control field coil 32 also on the direct axis of the amplidyne generator.

In the main D.C. motor the torque is proportional to the current flowing through the armature, and the armature current is a function of the applied voltage and the generated by the motor, said may oppose or may augment the applied voltage. The direction of the torque is determined by the direction of both the armature current flow and the flow of current through the motors field coil, but since the current flow through the field coil is a constant and the same direction the direction of the torque in the motor is determined by the armature current flow. The electromotive force developed by the motor is proportional to the speed of rotation of the armature with the polarity of this electromotive force reversing upon the reversal of the direction of rotation. In the motor-generator set circuit the direction of current flow is determined by the algebraic sum of the various voltages within the circuit and, consequently, it the electromotive force of the motor is greater than and opposes the applied voltage of the generator the current will flow from the motor of the main D.C. generator. The voltage output of the main D.C. generator, provided the speed of rotation of the armature is constant, is proportional in both magnitude and direction to the field of flux created by the field coil 26, said coil being in the amplidyne circuit.

In operating my invention the initial tension on the cable is established by adjusting the variable rheostat in the tension setting circuit until the motor is producing the desired torque output. This is brought about by the current flowing in the tension setting circuit and creating a certain field flux in the control field coil 30 of the amplidyne, and which amplidyne in turn causes a current proportional to the current in the tension setting circuit to flow through the field coil 26 of the main generator 15. The generator in turn produces a voltage and current, proportional to the current in the tension setting circuit, for driving the motor 16. At the equilibrium condition when the motor is not rotating and is producing enough torque to maintain the desired tension in the cable the motor is not generating an However, if the tension on the cable increases the torque of the motor is insufiicient to prevent paying out of the cable and which paying out of the cable in turn causes the motor to rotate and generate an supplementing the voltage applied to the motor by the generator. These two voltages are additive and increase the current flow in the armature of the motor, the torque output of the motor, and the tension in the cable. However, upon the rotation of the motor the tachometer generator 31 causes a current to flow in the tachometer circuit control field coil on the amplidyne, and which current establishes a field opposing the field established by the current flowing in the tension setting circuit control field coil on the amplidyne. The resulting control field of the amplidyne is less than when the motor is in a standstill state and therefor the voltage and current output of the amplidyne is decreased, causing the generators output voltage to drop, the motors output torque to drop, and the tension in the cable to become less. However, the voltage generated by the motor and the applied voltage of the generator complement each other causing sufiicient current to flow in the motor-generator set for the motor to produce the required torque to re-establish the desired tension in the cable.

Conversely, if the tension in the cable decreases the motor heaves-in creating an electromotive force opposing that generated by the main generator and therefore tending to decrease the torque output of the motor and the tento establish the desired tension on the cable.

ass see sion in the cable. However, the voltage output of the generator is increased until the difference in voltage between the generated voltage and the counter 18 again sufficient to cause enough current to flow in the motor to produce the necessary torque for maintaining desired tension in the cable. This is brought about by the tachometer generator causing a current to flow in the tachometer generator control field coil on the amplidyne to create a field which complements the field of the tension-setting circuit control field coil on the amplidyne. The result is an increase in the output of the amplidyne and a sufficient increase in the output voltage of the generator to overcome the counter generated by the motor and to cause the necessary current to flow in the armature of the motor for the motor to establish the desired tension in the cable.

A preferred embodiment of my invention in which the system maintains the cable around a certain position so as to limit the amount of cable that can be payed out and heaved-in is illustrated in Fig. 2. This embodiment comprises the elements of my previously described constant tension system plus an area control circuit 33, said circuit comprises a variable rheostat 34 having a fixed lead and a movable lead, a control field coil 35 on the load axis of the amplidyne 25, and a source of constant voltage such as a battery 36. The variable rheostat is coupled to the winch in order to directly vary the control field of the area control circuit on the amplidyne. In initially setting the system to maintain the cable around a certain position the area control circuit is disconnected from the system, the predetermined length of cable allowed to pay out and the tension-setting circuit adjusted Then, the area control circuit is connected into the system with the movable lead of the rheostat occupying practically the same position on the rheostat as the fixed lead in order to reduce the field in the area control field coil to substantially zero.

When the system is in operation and the tension in creases so as to pay out the cable the movable lead of the variable rheostat is gradually moved in accord with the movement of the winch to increase the field strength of the area control field on the amplidyne. This increase in the area control field adds to the control field of the tension-setting circuit on the amplidyne, but opposes the control field of the tachometer generator on the amplidyne, resulting in an increase in the voltage output of the amplidyne, an increase in the voltage output of the main D.C. generator, an increase in the torque of the motor and a heaving-in of the cable. With a heaving-in of the cable the movable lead is moved toward the fixed lead of the 'variable rheostat reducing the field in the control field coil and the torque of the motor. And, when the movable lead overlies the fixed lead the field strength in the control field coil is zero and the cable is the desired length. Conversely, when the tension in the cable decreases and the motor heaves-in the movable lead is moved to the other side of the fixed lead to set-up a field in the area control circuit field coil opposing both the field of the tensionsetting circuit control field coil and the field of the tachometer generator circuit field coil on the amplidyne. The opposition of the area control circuit field to these two fields tends to reduce the voltage output of the amplidyne, reduces the voltage output of the main generator, and the torque of the motor allowing the cable to pay out. The paying out of the cable moves the movable lead toward the fixed lead and when the two are juxtapositioned the field strength of the area control circuit field is zero and the cable is the desired length. As the resistance of the variable rheostat in the area control circuit changes gradually in response to the movement of the winch and therefore the voltage output varies slowly there is no hunting problem in this system.

A brief description concerning the schematic wiring diasigr'am in Fig. 3 and the actual wiring diagram in Fig. 4

' the automatic switch 65.

of actual shipboard installation of my constant tension system will be presented. The same reference numerals will be applied to the same members in each wiring diagram, and only those circuit components directly associated with the constant tension system will be identified by reference numerals.

This installation, adaptable for both manual and automatic tension control, comprises a prime mover in a 50 HP. AC. motor 37, a motor-generator set 38 having a 50 kw. D.C. generator 40 and a 36 HP. D.C. motor 41, an amplidyne 42, and an exciter generator 43 delivering a constant voltage output. Said 50 1-1.1. A.C. motor is driven by the ships power source and in turn drives the amplidyne, the constant potential generator, and the D.C. generator. The constant potential generator is set for a 250 volt output by adjusting a variable rheostat 44 in series with the generators field coil 45. Attached to the shaft 46 of the D.C. motor is a gear box 47 and leading out of the gear box is another shaft 48 connecting with a winch 50. And, winding around this winch is a cable 51.

This system is adaptable for either manual operation or automatic operation with the latter being either automatic tension control or the combination of automatic tension control and the maintaining of the cable around a certain position, i.e., maintaining the cable at a desired length. For manual operation contacts 52 in master switch 49 are closed so as to actuate contacts 53 and to place field coil 54 associated with the amplidyne generator 42 across the 250 volt line and in series with the resistances 55, 56 and 57. The closing of the contacts 53 also actuates brake relay 58 and thereby releases mo tor brake 60 to allow the D.C. motor to rotate. It is possible to increase the torque output of the motor by closing contacts 61, 62 and 63 in order to short out resistances 55, 56 and 57 and therefore to permit more current to flow through the control field coil 54 with a resulting greater output from both the amplidyne and the main generator. If it becomes necessary to pay out the cable the contacts 52 can be opened and contacts 64 in the master switch closed so as to again close contacts 53 and relay 58, releasing the brake and allowing the motor to operate. The control field is again placed across the 250 volt constant potential line and in series with the resistors 55, 56 and 57, but for the paying out of the cable the field is of the reverse polarity of the control field for the heaving-in of the cable. Therefore, the output of the amplidyne is decreased and there is a corresponding decrease in the voltage output of the DC. generator, the torque output of the motor, and the tension in the cable decreases with the same paying out.

The system is switched over to automatic operation, either for maintaining a constant tension in the cable or for both maintaining a constant tension in the cable and for maintaining the cable around a certain position, by opening the circuit across the master switch and closing The closing of this latter switch causes contactor 66 to close and which in turn closes brake relay 58 to release the motor brake. Also, the closing of the contactor 66 closes contactor 67 in tachometer generator armature circuit thereby placing the amplidynes control field 54 across the tachometer generators armature and, in addition, results in the closing of contacts 68 so as to place the amplidynes control field 70 across the 250 volt supply and in series with manual tension control rheostat 71 and automatic tension control rheostat 72. The relationship between the control fields 70 and 67 and their combined effect on the output of the amplidyne generator can be adjusted by controlling, with the aid of variable resistance 73, the field in the tachometer generators field coil 74.

In order to operate the system for maintaining only a constant tension in the cable, i.e., the length of the payed-out cable is not maintained the same at all times but is allowed to vary, the manual tension control rheostat 71 is adjusted until the current through the control field coil 70 on the amplidyne is sutficiently large so that the current output of the 50 kW. D.C. generator produces enough torque in the 35 H.P. DC. motor to balance the line of the winch and to develop the desired tension in the cable. If the cable should pay out and therefore the tension increase the tachometer generator 75 will develop a field in the control field coil 54 opposing the field in the control field coil 70. This results in a decrease in the amplidynes output, less torque output from the motor, and a reduction in the tension of the cable until the tension is again the desired tension. When the tension in the cable is less than the desired tension and the motor begins to heave-in and, as a result, the torque output of the motor decreases the tachometer generator develops a field in the control field coil 54 complementing the field in the control field coil 70. This increase in the control field increases the output of the amplidyne, the torque output of the motor, and the tension in the cable until the desired tension is reached.

The system can also be operated to both maintain a constant tension in the cable and to maintain the cable at a certain position. Such operation is brought about by first establishing the desired tension in the cable with the winch at a standstill state and then closing clutch control contacts 76 which engages the automatic tension control. If the load on the cable increases and the same begins to pay out and the tension increases the tachometer generator will operate as described above in regard to manual control operation but the automatic tension control will act to increase the field in the control field coil 70, and thereby increase the output of the amplidyne and the DC. motor. The tension in the cable remains greater than the desired tension until a sufficient length of said cable has been heaved-in and the cable is again at the predetermined certain position. Conversely, if the pull on the cable decreases and the motor heaves-in the tachometer generator develops a field tending to increase the output of the amplidyne but, this time, the tension control acts to decrease the current through the field coil 70 and to decrease the torque output of the motor. Until the cable is around the predetermined certain position the torque output of the motor remains less than required to establish the desired tension in the cable. When the cable is around that position the torque output of the motor is again suflicient to establish the desired tension.

If at any time it is necessary to remove the system from automatic operation contacts 76 are opened to deenergize reset relay 77 which in turn opens contactors 78 and thereby removes both the manual and the automatic control sections from the 250 volt constant potential supply. The opening of the contacts 76 also drops out the relays 66, 58 and 80 and which removes all excitation from the generating system and sets the brake to bring the winch to a stop. Circuit breaker 81 is a circuit disconnect circuit, and field coil 82in the main loop circuit of the motor generator set, is placed in opposition to the flux developed by the field coil 83 of the main generator so as to make the generator self-regulating.

Although express reference has been made to an amplidyne circuit in the preceding description of my invention as the control means for controlling the voltage applied to the driving motor, it is to be understood that this has been for ease of description and is not intended to be a limitation of the invention as there are other controls capable of controlling the voltage applied to the driving motor and, therefore, maintaining a substantially uniform current flow in the motors armature and a constant torque output. Exemplary of such other control means are a rototrol, and an electronic control like grid-controlled thyratrons. In employing a rototrol in stead of an amplidyne for regulating the voltage output of the DC. generator the rototrol is driven by a separate motor. A pattern or reference field is established in one of the rototrols field coils by the tension setting circuit and another field, either opposing or enhancing the previously mentioned field, in one of the rototrols other field coils is established by the tachometer generator or a pilot generator connecting with the motor. As the rototrol controls the field in the generators field coil the rotation of the motors rotor regulates the output of the tachometer generator and thereby increases or decreases the output of the rototrol, the voltage output of the main generator, and re-establishes the desired torque in the motor. Grid-controlled thyratrons are also employed to regulate the field in the main D.C. generators field coils and thereby to control the voltage output of the generator, the armature current of the driving motor and the motors torque output. The grid bias of the thyratron, and consequently the output of the tube and the field on the generator, is established by the tension-setting circuit with the tachometer generator superimposing a voltage on the grid to control the firing of the tube and the field in the generators field coil. In actual use the thyratrons are in a push-pull arrangement to give a more uniform control. In certain instances, namely, those instances when the driving motor is small and the power required is not large, it is possible to eliminate the main D.C. generator employed for driving the motor and, instead, to drive the motor directly by the control means. Having the foregoing explanation of possible variants in mind it is to be understod that the term amplidyne as used in the description and in the claims is construed in a generic sense as meaning and including the referred to and other like or suitable controls.

The advantages of the invention, it is thought, will have been clearly understood from the foregoing detailed description of the illustrated preferred embodiments. Minor changes will suggest themselves and may be resorted to without departing from the spirit of the invention, wherefore it is my intention that no limitations be implied and that the hereto annexed claims be given a scope fully commensurate with the broadest interpretation to which the employed language admits.

I claim:

1. In a system for maintaining a constant tension in a first means subject to pulling forces and for maintaining the first means about a certain position, a motor-generator couple, said couple comprising a motor and a main generator, said generator having a field coil, an amplidyne generator connected with the field coil for varying a field established by the field coil, said amplidyne generator having control field coils, a second means connecting with at least one of the control field coils and responsive to the motor for varying a resultant field established by an unvarying reference control field coil, and a third means connecting with at least one of the control field coils and responsive to the position of the first means for varying a field established by the control field coils in order to maintain said first means about said certain position.

2. A constant-tension system for maintaining both a constant tension in a first means subject to pulling forces and the first means about a reference position, said system comprising a motor-generator couple, means operatively connecting said first means with the motor, a second means for maintaining a substantially constant voltage difference between the generator and the motor, said second means having a field coil for the generator, a third means for establishing an unvarying reference field in said field coil, a fourth means responsive to the tension in the first means and acting in unison with the third means to establish a field in the field coil to maintain a substantially constant voltage difference between the generator and the motor and a constant tension in the first means, and a fifth means responsive to the position of the first means, said fifth means upon the moving of the first means from the reference position creating a field in the first coil to cause the generator to drive the motor until the first means is about said reference position.

.3. A constant-tension system for maintaining both a aseaeoa constant tension in a first means subject to pulling forces and the first means about a reference position, said system comprising a motor-generator couple, means for connecting said motor with the first means for heaving-in and paying-out the same, a second means for maintaining a substantially constant voltage difference between the generator and the motor, said second means including a field coil for the generator, tension-setting circuit means operatively connected with said field coil for establishing an unvarying reference field in said field coil, tachometer generator means operatively connected with said field coil and responsive to the motor and the tension in the first means for varying the field in said field coil to maintain a substantially constant voltage difference between the generator and the motor and a constant tension in the first means, and a position-setting circuit responsive to the position of the first means, whereby said positionsetting circuit upon the moving of the first means from the reference position varies the field in the field coil to cause the generator to drive the motor until the first means is approximately at said reference position.

4. A constant-tension system for maintaining a constant tension in a first means subject to pulling forces, said system comprising a motor-generator couple, means connecting said motor with the first means for heaving-in and paying-out the same, a field coil for the generator, an amplidyne generator connecting with the field coil, tension-setting circuit means independent of said motorgenerator couple acting through the amplidyne generator for establishing an unvarying reference field in said field coil, a tachometer generator responsive to the motor and the tension in the first means and operating through the amplidyne generator to alter the field in said field coil to maintain a substantially constant voltage difference between the generator and the motor and a constant tension in the first means, and position-setting circuit means responsive to the position of the first means for creating upon the moving of the first means from the reference position, and operating through the amplidyne generator, a field in the field coil causing the generator to drive the motor until the first means is positioned approximately at said reference position.

5. In a system for maintaining a tension in a first means subject to pulling forces and for maintaining the first means about a certain position, a motor operatively connected to the first means, a generator for driving the motor, a second means independent of the generator to motor connection for establishing unvarying reference field with respect to the generator, a third means for altering said reference field, said third means being capable of establishing a field to augment the reference field and also to oppose the reference field, said third means being responsive to the torque of the motor and the tension in the first means, a fourth means for altering said reference field, said fourth means being capable of establishing a field to augment the reference field and also to oppose the reference field, and said fourth means being responsive to the position of the first means so as to maintain the same approximately at a certain position.

References Cited in the file of this patent UNITED STATES PATENTS 2,165,111 Rasmussen July 4, 1939 2,508,180 Mahnke et al. May 16, 1950 2,523,085 Allbert et al. Sept. 19, 1950 2,607,908 Edwards et al. Aug. 19, 1952 2,688,111 Jones Aug. 31, 1954 FOREIGN PATENTS 603,723 Great Britain June 22, 1948 

